The development of N-methyl-D-aspartate receptors in cat visual cortex

N-methyl-D-aspartate receptors strongly regulate postsynaptic activity levels during optic nerve regeneration

During development, neuronal activity is used as a cue to guide synaptic rearrangements to refine connections. Many studies, especially in the visual system, have shown that the N-methyl-D-aspartate receptor (NMDAr) plays a key role in mediating activity-dependent refinement through long-term potentiation (LTP)-like processes. Adult goldfish can regenerate their optic nerve and utilize neuronal activity to generate precise topography in their projection onto tectum. Although the NMDAr has been implicated in this process, its precise role in regeneration has not been extensively studied. In examining NMDAr function during regeneration, we found salient differences compared with development. By using field excitatory postsynaptic potential (fEPSP) recordings, the contribution of the NMDAr at the primary optic synapse was measured. In contrast to development, no increase in NMDAr function was detectable during synaptic refinement. Unlike development, LTP could not be reliably elicited during regeneration. Unexpectedly, we found that NMDAr exerted a major effect on regulating ongoing tectal (postsynaptic) activity levels during regeneration. Blocking NMDAr strongly suppressed spontaneous activity during regeneration but had no significant effect in the normal projection. This difference could be attributed to an occlusion effect of strong optic drive in the normal projection, which dominated ongoing tectal activity. During regeneration, this optic drive is largely absent. Optic nerve stimulation further indicated that the NMDAr had little effect on the ability of optic fibers to evoke early postsynaptic impulse activity but was important for late network activity. These results indicate that, during regeneration, the NMDAr may play a critical role in the homeostatic regulation of ongoing activity and network excitability.

Energy metabolism in mammalian brain during development

Production of energy for the maintenance of ionic disequilibria necessary for generation and transmission of nerve impulses is one of the primary functions of the brain. This review attempts to link the plethora of information on the maturation of the central nervous system with the ontogeny of ATP metabolism, placing special emphasis on variations that occur during development in different brain regions and across the mammalian species. It correlates morphological events and markers with biochemical changes in activities of enzymes and pathways that participate in the production of ATP. The paper also evaluates alterations in energy levels as a function of age and, based on the tenet that ATP synthesis and utilization cannot be considered in isolation, investigates maturational profiles of the key processes that utilize energy. Finally, an attempt is made to assess the relevance of currently available animal models to improvement of our understanding of the etiopathology of various disease states in the human infant. This is deemed essential for the development and testing of novel strategies for prevention and treatment of several severe neurological deficits.

Experience-dependent changes in NMDAR1 expression in the visual cortex of an animal model for amblyopia

When normal binocular visual experience is disrupted during postnatal development, it affects the maturation of cortical circuits and often results in the development of poor visual acuity known as amblyopia. Two main factors contribute to the development of amblyopia: visual deprivation and reduced binocular competition. We investigated the affect of these two amblyogenic factors on the expression of the NMDAR1 subunit in the visual cortex because activation of the NMDA receptor is a key mechanism of developmental neural plasticity. We found that disruption of binocular correlations by monocular deprivation promoted a topographic loss of NMDAR1 expression within the cortical representations of the central visual field and the vertical and horizontal meridians. In contrast, binocular deprivation, which primarily affects visual deprivation, promoted an increase in NMDAR1 expression throughout the visual cortex. These different changes in NMDAR1 expression can be described as topographic and homeostatic plasticity of NMDA expression, respectively. In addition, the changes in NMDA expression in the visual cortex provide a greater understanding of the neural mechanisms that underlie the development of amblyopia and the potential for visual recovery.

Synaptic and Molecular Mechanisms Regulating Plasticity during Early Learning

Many behaviors are learned most easily during a discrete developmental period, and it is generally agreed that these "sensitive periods" for learning reflect the developmental regulation of molecular or synaptic properties that underlie experience-dependent changes in neural organization and function. Avian song learning provides one example of such temporally restricted learning, and several features of this behavior and its underlying neural circuitry make it a powerful model for studying how early experience sculpts neural and behavioral organization. Here we describe evidence that within the basal ganglia-thalamocortical loop implicated in vocal learning, song acquisition engages N-methyl-d-aspartate receptors (NMDARs), as well as signal transduction cascades strongly implicated in other instances of learning. Furthermore, NMDAR phenotype changes in parallel with developmental and seasonal periods for vocal plasticity. We also review recent studies in the avian song system that challenge the popular notion that sensitive periods for learning reflect developmental changes in the NMDAR that alter thresholds for synaptic plasticity.

Laminar differences in plasticity in area 17 following retinal lesions in kittens or adult cats

Circumscribed retinal lesions in adult cats result in a reorganization of circuitry in area 17 such that neurons in the lesion projection zone (LPZ) can now be activated, not from their original receptive fields (RFs) but from regions of normal retina adjacent to the lesion ('ectopic' RFs). We have studied this phenomenon further by making circumscribed monocular retinal lesions in 8-week-old kittens and recording responses to visual stimuli of neurons in the LPZ of area 17 when these cats reached adulthood. These responses have been compared with those in adult-lesioned cats either of relatively short postlesion survival (2-24 weeks) or long postlesion survival (3.5-4.5 years). In both kitten-lesioned and adult-lesioned animals most LPZ neurons recorded from the supragranular layers (II and III) not only exhibited new ectopic RFs when stimuli were presented via the lesioned eye but the RF properties (e.g. the sizes of excitatory RFs, orientation and direction selectivities, velocity preferences and upper cut-off velocities) were often indistinguishable from those seen when stimuli were presented via the nonlesioned eye. Similarly, in both kitten-lesioned and adult-lesioned animals, most LPZ neurons recorded from the granular and infragranular layers (IV, V, VI), like those recorded from the supragranular layers, were binocular. However, in adult-lesioned but not in kitten-lesioned animals, the responses and the upper cut-off velocities of LPZ cells recorded from the granular and infragranular layers to stimuli presented via ectopic RFs tended to be, respectively, substantially weaker and lower than those for stimuli presented via the nonlesioned eye. The age-related laminar differences in reorganizational plasticity of cat striate cortex correlate with the lamino-temporal pattern of distribution of N-methyl-d-aspartate glutamate receptors in striate cortex.

Postnatal phencyclidine-induced deficit in adult water maze performance is associated with N-methyl-D-aspartate receptor upregulation

The N-methyl-D-aspartate (NMDA) receptor plays an important role in developmental plasticity. Earlier, we have shown that blocking the NMDA receptor with the non-competitive antagonist phencyclidine (PCP), during a brief postnatal period, disrupts the water maze performance in young juvenile rats (starting at 25 days of age). We now show the long-term effects of postnatal phencyclidine exposure on spatial learning and memory. Male and female rats were exposed to PCP (1 and 5mg/kg) or saline, from postnatal days 5-15, and their performance in the Morris water maze (MWM) was tested both as adolescents (starting on postnatal day (PD) 35) and as adults (starting on postnatal day 60). Separate groups of adult male and female postnatal PCP-treated and saline-treated rats were sacrificed and saturation [3H]MK-801 binding experiments were carried out in their hippocampi and frontal cortices; hippocampus and frontal cortex have high densities of NMDA receptors and both regions are important in spatial learning and memory. Postnatal PCP administration disrupted the water maze performance both in adolescent and adult rats of both sexes. Adult male and female rats treated postnatally with PCP had increased maximal [3H]MK-801 binding in the hippocampus and frontal cortex compared to same-sex saline-treated controls. Taken together, repeated postnatal PCP (RPP) administration impaired the acquisition of spatial learning in adolescent and adult male and female rats, and this cognitive deficit was associated with increased [3H]MK-801 labeled NMDA receptor in the hippocampus and frontal cortex. These findings are consistent with the hypothesis that PCP treatment during the postnatal period produces deficits in the water maze performance by disrupting the developing glutamatergic system.

Separable features of visual cortical plasticity revealed by N-methyl-D-aspartate receptor 2A signaling

How individual receptive field properties are formed in the maturing sensory neocortex remains largely unknown. The shortening of N-methyl-d-aspartate (NMDA) receptor currents by 2A subunit (NR2A) insertion has been proposed to delimit the critical period for experience-dependent refinement of circuits in visual cortex. In mice engineered to maintain prolonged NMDA responses by targeted deletion of NR2A, the sensitivity to monocular deprivation was surprisingly weakened but restricted to the typical critical period and delayed normally by dark rearing from birth. Orientation preference instead failed to mature, occluding further effects of dark rearing. Interestingly, a full ocular dominance plasticity (but not orientation bias) was selectively restored by enhanced inhibition, reflecting an imbalanced excitation in the absence of NR2A. Many of the downstream pathways involved in NMDA signaling are coupled to the receptor through a variety of protein-protein interactions and adaptor molecules. To further investigate a mechanistic dissociation of receptive field properties in the developing visual system, mice carrying a targeted disruption of the NR2A-associated 95-kDa postsynaptic density (PSD95) scaffolding protein were analyzed. Although the development and plasticity of ocular dominance was unaffected, orientation preference again failed to mature in these mice. Taken together, our results demonstrate that the cellular basis generating individual sensory response properties is separable in the developing neocortex.

The Involvement of N-Methyl-D-Aspartate Receptors in Induction and Maintenance of Long-Term Potentiation in Rat Visual Cortex

Pyramidal neurons from layers II and III of rat visual cortex slices were studied with intracellular recordings. The involvement of N-methyl-D-aspartate (NMDA) receptors was investigated: (1) in the synaptic response to white matter stimulation; (2) in the induction of long-term potentiation (LTP); and (3) in the maintenance of LTP. Bath application of 25 microM of 2-amino-5-phosphonovalerate (APV), an NMDA receptor antagonist, caused a slight (< 10%) reduction of the amplitude of the synaptic response elicited by white matter stimulation. The APV-sensitive excitatory postsynaptic potential (EPSP) had a longer peak latency and duration than the APV-resistant EPSP. Bath application of 10 microM of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), a non-NMDA glutamate receptor antagonist, revealed a CNQX-resistant EPSP in response to white matter stimulation which was APV-sensitive. The time course of the CNQX-resistant EPSP was similar to that of the APV-sensitive EPSP and its onset latency was similar to that of the synaptic response in normal medium. Bath application of the GABA-A antagonist bicuculline (0.1 to 0.5 microM) led to a progressive enhancement of the amplitude of the APV-sensitive EPSP. At bicuculline concentrations above 0.3 microM the amplitude of this EPSP increased with membrane depolarization as was the case for the CNQX-resistant EPSP implying that the NMDA receptors were located on the recorded neuron. The susceptibility of the cells to undergo LTP was tested at various concentrations of bicuculline. The effectiveness of bicuculline treatment was quantified by comparing the amplitudes of the synaptic response to just subthreshold stimuli at two post-stimulus delays: (i) at 22 ms, which corresponds to the time to peak of both the initial inhibitory postsynaptic potential and the APV-sensitive EPSP; and (ii) at 8 - 11 ms post-stimulus, which corresponds to the peak of the postsynaptic potential (PSP) in normal medium. Bath application of APV, 20 min after the conditioning tetanus, allowed the authors to measure the amplitude of the APV-sensitive EPSP in the potentiated response. In normal medium, the ratio of the late over the early PSP amplitude was 33.6 +/- 4.1% and tetanic stimulation failed to induce LTP. The conditions remained the same at bicuculline concentrations of 0.1 to 0.2 microM. At higher concentrations of bicuculline the amplitude ratio of late versus early PSP increased and tetanic stimulation induced LTP. In cells, in which bicuculline had caused small ratio increases, only the APV-sensitive EPSP underwent LTP. In cells in which bicuculline had caused large ratio changes, both the APV-resistant and the APV-sensitive EPSP showed LTP. Together with the previous finding that blockade of NMDA receptors prevents LTP (Artola and Singer, 1987) these results suggest that there is a threshold for LTP induction, which is only reached if NMDA receptor-gated channels are sufficiently activated. The data indicate further that the NMDA receptor-mediated EPSP is itself susceptible to LTP whereby its LTP threshold is lower than that of the APV-resistant EPSP. Given the different LTP thresholds of the APV-resistant and APV-sensitive EPSPs, the possibility is raised that their potentiation depends on different mechanisms.

Transient Sensitivity of Rat Cerebellar Purkinje Cells to N-methyl-D-aspartate during Development. A Voltage Clamp Study in in vitro Slices

In vitro sagittal slices of immature rat cerebellum were used to study the development of the sensitivity of Purkinje cells (PC) to L-glutamate (Glu) and N-methyl-D-aspartate (NMDA). In 8-day-old animals, all PCs recorded in magnesium-free medium responded to iontophoretic applications of both agonists by transient and dose dependent inward currents which, in both cases, were heavily contaminated by a Glu and NMDA-induced synaptic noise. When 5 x 10-6 M tetrodotoxin (TTX) was added to the perfusing medium, this evoked synaptic noise was completely abolished in most cells whereas clear-cut inward currents induced in PCs by Glu and NMDA applications on their dendrites were still visible. These responses were selectively antagonized by the non-NMDA glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and by the NMDA receptor antagonist D-2-aminophosphono-5-valeric acid (2APV) respectively. Excitatory responses induced by aspartate in 8 - 10-day-old PCs were also markedly antagonized by CNQX. At this stage, the sensitivity of PCs to NMDA was about one order of magnitude less than that to Glu. In 15 - 20-day-old animals, all PCs were still responsive to Glu whereas only 70% of them were still excited by NMDA in the presence of TTX in the bath. Furthermore, the sensitivity of PCs to Glu was higher than at 8 days of age, whereas that to NMDA was significantly lower, even when considering only those cells which still responded to this agonist. This trend was still accentuated later on since at 2 months of age, only 25% of PCs were excited by NMDA whereas their sensitivity to Glu was similar to that observed in 15 - 20-day-old animals. Therefore, the present results are fully consistent with the view that PCs have a transient expression of NMDA receptors during development.

Development of NMDA and non-NMDA receptor-mediated excitatory synaptic transmission in geniculocortical and corticocortical connections in organotypic coculture preparations

Development of N-methyl-D-aspartate (NMDA) and non-NMDA receptor-mediated excitatory synaptic transmission was studied in the visual cortex using organotypic slice cocultures. A slice of visual cortex (VC) dissected from newborn rats was cocultured with either a chunk of embryonic lateral geniculate nucleus (LGN) or another VC. During 7-38 days in vitro (DIV), geniculocortical monosynaptic excitatory postsynaptic potentials (EPSPs) were recorded from layer IV neurons in response to stimulation of the LGN in cocultures of the VC with the LGN. Similarly, corticocortical monosynaptic EPSPs were recorded from layers II/III and V/VI neurons in cocultures of two VCs when stimulating the partner VC. The initial slopes of the non-NMDA and NMDA receptor-mediated components of the EPSPs, which were dissociated pharmacologically, were assessed and compared among three different culture stages, early (7-11 DIV), middle (12-15 DIV) and late (17-38 DIV). With progression of the culture stage, the non-NMDA component tended to increase in both the geniculocortical and corticocortical connections. In contrast, the NMDA component exhibited distinct developmental changes. The NMDA component in layer IV neurons, which receive geniculate inputs, showed a transient increase in the middle stage. In the corticocortical connection, the magnitude of the NMDA component was large in the early stage and maintained through all culture stages in layer V/VI cells, whereas in layer II/III cells it decreased sharply by the late stage. Our results suggest that glutamatergic transmission in the visual cortex develops differently in the geniculocortical and corticocortical connections.

Early sensory and hormonal experience modulate age-related changes in NR2B mRNA within a forebrain region controlling avian vocal learning

Male zebra finches are most apt to mimic songs heard between posthatch days (PHD) 35 and 65, and this vocal learning depends, in part, on the activation of N-methyl-D-aspartate receptors (NMDAR) within a discrete forebrain circuit that includes the lateral magnocellular nucleus of the anterior neostriatum (lMAN) and area X. Using in situ hybridization, we show that transcripts for both the constitutive NMDAR subunit NR1 and the modulatory subunit NR2B decrease abruptly in the lMAN between PHD20 and 40. This downregulation corresponds to the onset of song learning and a transition from slow to faster NMDAR currents in lMAN neurons. In area X, NR1 mRNA increases as NR2B mRNA decreases during song development. To understand how these changes in NMDAR mRNA might regulate song learning, we next investigated how manipulations that influence song development affect NMDAR mRNA expression. Early isolation from conspecific song (which delays closure of the sensitive period for song learning) selectively increases NR2B, but not NR1 mRNA, within lMAN at PHD60. In contrast, exposure to testosterone beginning at PHD20 (which impairs song development and hastens the developmental transition to faster NMDAR current kinetics within lMAN) accelerates the decline in NR2B mRNA in lMAN, again without affecting NR1 transcript levels. Neither manipulation significantly effects NR1 or NR2B mRNA levels in area X. Our data suggest that developmental changes in the expression of specific NMDAR subunits may regulate periods of neural and behavioral plasticity and that flexibility in the timing of these sensitive periods may be achieved through experience and/or hormone-dependent modulation of NMDAR gene expression.

Primed-burst potentiation in adult rat visual cortex in vitro

The effectiveness of &theta; pattern primed-bursts (PBs) on development of PB potentiation was investigated in layer II/III of the adult rat visual cortex in vitro. Experiments were carried out in the visual cortical slices. Population excitatory postsynaptic potentials (pEPSPs) were evoked in layer II/III by stimulation of either white matter or layer IV. To induce long-term potentiation (LTP), eight episodes of PBs were delivered at 0.1 Hz. Regardless of stimulation site, field potential recorded in layer II/III consisted of two components: a short latency and high amplitude response called pEPSP1, and a long latency and low amplitude response called pEPSP2. The incidence of LTP produced by PBs of layer IV was higher than that of the white matter tetanization. In contrast, PBs of both layer IV and white matter reliably produced LTP of pEPSP2 in layer II/III. It is concluded that PBs, as a type of activity pattern, of either white matter or layer IV can gain access to the modifiable synapses that are related to pEPSP2 in layer II/III, but accessibility of the modifiable synapses that are related to pEPSP1 depends on tetanization site. Relevancy of the results to the plasticity gate hypothesis is also discussed.

Mechanisms underlying epileptogenesis in cortical malformations

The presence of developmental cortical malformations is associated with epileptogenesis and other neurological disorders. In recent years, animal models specific to certain malformations have been developed to study the underlying epileptogenic mechanisms. Teratogens (chemical, thermal or radiation) applied during cortical neuroblast division and migration result in lissencephaly and focal cortical dysplasia. Animals with these malformations have a lowered seizure threshold as well as histopathologies typical of those found in human dysgenic brains. Alterations that may promote epileptogenesis have been identified in lissencephalic brains, such as increased numbers of bursting types of neurons, and abnormal connections between hippocampus, subcortical heterotopia, and neocortex. A distinct set of pathological properties is present in animal models of 4-layered microgyria, induced with cortical lesions made during late stages of cortical neuroblast migration. Hyperexcitability has been demonstrated in cortex adjacent to the microgyrus (paramicrogyral zone) in in vitro slice preparations. A number of observations suggest that cellular differentiation is delayed in microgyric brains. Other studies show increases in postsynaptic glutamate receptors and decreases in GABA(A) receptors in microgyric cortex. These alterations could promote epileptogenesis, depending on which cell types have the altered receptors. The microgyrus lacks thalamic afferents from sensory relay nuclei, that instead appear to project to the paramicrogyral region, thereby increasing excitatory connectivity within this epileptogenic zone. These studies have provided a necessary first step in understanding molecular and cellular mechanisms of epileptogenesis associated with cortical malformations.

Enhanced NR2A subunit expression and decreased NMDA receptor decay time at the onset of ocular dominance plasticity in the ferret

Enhanced NR2A subunit expression and decreased NMDA receptor decay time at the onset of ocular dominance plasticity in the ferret. The NMDA subtype of glutamate receptor is known to exhibit marked changes in subunit composition and functional properties during neural development. The prevailing idea is that NMDA receptor-mediated synaptic responses decrease in duration after the peak of cortical plasticity in rodents. Accordingly, it is believed that shortening of the NMDA receptor-mediated current underlies the developmental reduction of ocular dominance plasticity. However, some previous evidence actually suggests that the duration of NMDA receptor currents decreases before the peak of plasticity. In the present study, we have examined the time course of NMDA receptor changes and how they correlate with the critical period of ocular dominance plasticity in the visual cortex of a highly binocular animal, the ferret. The expression of NMDA receptor subunits NR1, NR2A, and NR2B was examined in animals ranging in age from postnatal day 16 to adult using Western blotting. Functional properties of NMDA receptors in layer IV cortical neurons were studied using whole cell patch-clamp techniques in an in vitro slice preparation of ferret primary visual cortex. We observed a remarkable increase in NR1 and NR2A, but not NR2B, expression after eye opening. The NMDA receptor-mediated synaptic currents showed an abrupt decrease in decay time concurrent with the increase in NR2A subunit expression. Importantly, these changes occurred in parallel with increased ocular dominance plasticity reported in the ferret. In conclusion, molecular changes leading to decreased duration of the NMDA receptor excitatory postsynaptic current may be a requirement for the onset, rather than the end, of the critical period of ocular dominance plasticity.

Spatiotemporal patterning of glutamate receptors in developing ferret striate cortex

We have studied glutamate receptor levels during very early phases of cortical formation by using quantitative in vitro autoradiography to map the expression of NMDA, AMPA and kainate receptors in the developing primary visual cortex of the ferret. NMDA and non-NMDA receptors exhibit very different developmental profiles in primary visual cortex. NMDA receptor density is low at birth and increases throughout the first 2 postnatal months, rising between threefold (layers II/III) and ninefold (layer VI). In contrast, AMPA receptors are abundant at birth and their density remains constant for the first postnatal month, before rising by a maximum of 1.7-fold (layer I) at around the time of eye-opening (postnatal day 32). Kainate receptors are also present in high levels at birth and their expression levels rise in the early postnatal period by between 1. 5-fold (layer I) and threefold (layers V/VI) to a peak just after eye-opening. The proportion of the total ionotropic glutamate receptor binding contributed by NMDA receptors thus rises from 5% at birth to a maximum of 22% at 2 months of age, while the AMPA receptor contribution falls from 87% to 72% over the same period. Below cortex, all three glutamate receptor subtypes are expressed in the subplate region for the first 3 postnatal weeks. These developmental patterns, combined with the fact that AMPA receptors are densely expressed in the proliferative zones underlying presumptive area 17, indicate that non-NMDA receptor expression levels in primary visual cortex are mostly specified much earlier than those of NMDA receptors.

Relationship between dendritic pruning and behavioral recovery following sensorimotor cortex lesions

A unilateral injury to the forelimb area of the sensorimotor cortex results in an increase in dendritic arborization in the contralateral homotopic cortex which is followed by a pruning back of these dendritic arbors. The increase in arborization is due to an increase in the use of the unimpaired forelimb for postural-motor support; whereas, the dendritic pruning is related, in time, to the return to more symmetrical limb use, but is not prevented by the maintenance of asymmetrical limb use. Dendritic pruning can be prevented by administering an NMDA receptor antagonist (such as MK801 or ethanol) during the pruning phase. This manipulation also coincides with the chronic reinstatement of behavioral deficits. The purpose of this study was to see whether removing the antagonism of the NMDA receptor results in the eventual return of dendritic pruning and behavioral recovery. Therefore, MK801 was administered to lesioned animals starting at post-lesion day 18. One group received MK801 injections until day 60 (Lesion + MK60) and another lesioned group received MK801 until day 30 after which the injections were changed to saline until day 60 (Lesion + MK30). Lesion + MK60 animals showed a prevention of dendritic pruning as well as a chronic reinstatement of forelimb deficits. Lesion + MK30 animals also showed a prevention of dendritic pruning, however, they showed behavioral recovery. These findings suggest that pruning of dendritic arbors may not be directly related to behavioral recovery.

Localized contribution of N-methyl-D-aspartate receptors to synaptic input-induced rise of calcium in apical dendrites of layer II/III neurons in rat visual cortex

To examine the role of N-methyl-D-aspartate receptors in the synaptic input-induced increase in Ca2+ in layer II/III neurons of visual cortex, Ca2+ imaging with a relatively low-affinity indicator was carried out simultaneously with whole-cell recordings of synaptic activity in cortical slices of young rats. Electrical stimulation of layer IV elicited excitatory postsynaptic potentials that generated action potentials concomitantly with a marked rise of Ca2+ signal in apical dendrites of 24 pyramidal cell-like neurons. Dendritic regions about 50 microm from the soma (peak regions) consistently showed the strongest increase in signal. The application of an N-methyl-D-aspartate receptor antagonist, DL-2-amino-5-phosphonovalerate, reduced the Ca2+ signal in peak regions but did not in other regions in nine of the 15 neurons tested. In all the dendritic regions in which Ca2+ signal was measured, the magnitude of the increase in signal was related linearly to the number of action potentials, suggesting that voltage-dependent Ca2+ channels activated by action potentials may also be involved in the increase. In 25 of 33 neurons which were voltage-clamped at 80 mV and perfused with the Mg2+-free solution, layer IV stimulation could induce a local rise in Ca2+ signal in apical dendrites. This rise was blocked by the N-methyl-D-aspartate receptor antagonist almost completely. Such an antagonist-sensitive Ca2+ rise was still seen even when the membrane potential was held at +10 mV when voltage-dependent Ca2+ channels were expected not to be activated by excitatory synaptic inputs. These results suggest that N-methyl-D-aspartate receptors are involved in the synaptic input-induced rise in Ca2+ at postsynaptic sites in apical dendrites of layer II/III neurons, and the increase may be boosted through voltage-dependent Ca2+ channels activated by action potentials.

Patchy distribution of NMDAR1 subunit immunoreactivity in developing visual cortex

Development of ocular dominance columns is dependent on patterned retinal activity, and yet patterned activity alone cannot explain all aspects of cortical column development. Features intrinsic to the cortex have been proposed to interact with activity to guide the patterning of cortical columns (), and the NMDA receptor, because of its role in experience-dependent plasticity, is an obvious candidate. Using immunohistochemical techniques, we found a transiently patchy distribution of the NMDA receptor 1 (NMDAR1) subunit in kitten visual cortex. Regularly spaced patches of NMDAR1-immunoreactive neurons were found at the top of the cortical plate in the developing visual cortex at 2 weeks of age. At 4-5 weeks of age, the radial extent of the NMDAR1 patches spanned the supragranular layers, and by 12 weeks of age, this nonuniform pattern of NMDAR1 immunostaining was no longer apparent. Monocular visual experience prevented the expression of the NMDAR1 patches, but just 4 d of subsequent binocular visual experience was sufficient to promote expression of the patches. Furthermore, the NMDAR1 patches tended to be associated with the borders of ocular dominance columns. These results suggest that the degree of plasticity associated with NMDA-mediated mechanisms is elevated in local regions across the tangential extent of the visual cortex and that the NMDAR1 patches may participate in sculpting the overall arrangement of visual cortical columns.

Pharmacological extension of the sensitive period for imprinting in Gallus domesticus

Precocial animals, such as the chick, exhibit a form of learning termed filial imprinting. The chick's sensitive period for filial imprinting is restricted to the first few days after hatching. The neural mechanism that terminates the sensitive period is not fully understood. It is thought to be an experience-dependent event because once a chick has imprinted, it will not readily imprint on another stimulus. However, even dark-reared chicks eventually lose the ability to imprint, which suggests that the ending of the sensitive period may not be entirely experience-dependent. The present study investigates factors that may contribute to the ending of the sensitive period. In our experiments, dark-reared chicks were unable to imprint after Day 2 posthatching, but chicks treated 10 h after hatching with an intramuscular injection of the noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist ketamine (55 mg/kg) and the alpha2-adrenoceptor agonist xylazine (6 mg/kg) (KX) imprinted on a stuffed hen 8 days after hatching. Similarly treated chicks did not imprint on a red and black box, although the box was an effective imprinting stimulus for Day 2 chicks. Chicks treated with KX at 20 or 40 h posthatching or on Day 4 or 7 as well as controls treated with pyrogen-free saline were unable to imprint on Day 8.

Activity-dependent regulation of NMDAR1 immunoreactivity in the developing visual cortex

NMDA receptors have been implicated in activity-dependent synaptic plasticity in the developing visual cortex. We examined the distribution of immunocytochemically detectable NMDAR1 in visual cortex of cats and ferrets from late embryonic ages to adulthood. Cortical neurons are initially highly immunostained. This level declines gradually over development, with the notable exception of cortical layers 2/3, where levels of NMDAR1 immunostaining remain high into adulthood. Within layer 4, the decline in NMDAR1 immunostaining to adult levels coincides with the completion of ocular dominance column formation and the end of the critical period for layer 4. To determine whether NMDAR1 immunoreactivity is regulated by retinal activity, animals were dark-reared or retinal activity was completely blocked in one eye with tetrodotoxin (TTX). Dark-rearing does not cause detectable changes in NMDAR1 immunoreactivity. However, 2 weeks of monocular TTX administration decreases NMDAR1 immunoreactivity in layer 4 of the columns of the blocked eye. Thus, high levels of NMDAR1 immunostaining within the visual cortex are temporally correlated with ocular dominance column formation and developmental plasticity; the persistence of staining in layers 2/3 also correlates with the physiological plasticity present in these layers in the adult. In addition, visual experience is not required for the developmental changes in the laminar pattern of NMDAR1 levels, but the presence of high levels of NMDAR1 in layer 4 during the critical period does require retinal activity. These observations are consistent with a central role for NMDA receptors in promoting and ultimately limiting synaptic rearrangements in the developing neocortex.

Age-related functional changes of the glutamate receptor channels in rat Meynert neurones

1. The developmental changes of glutamate receptors (GluRs) in acutely dissociated rat Meynert neurones were investigated using the conventional whole cell and nystatin perforated patch recording modes under voltage-clamp conditions. 2. The neurones became less responsive to N-methyl-D-aspartic acid (NMDA) with age, most dramatically between 1 day and 2 weeks, while the responses to kainic acid (KA) and L-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) gradually increased. The metabotropic GluR response appeared a few days after birth, but thereafter no further change was observed. 3. The decrease in the NMDA response during postnatal development was due to an abrupt reduction in the number of receptors without affecting the affinity, voltage-dependent Mg2+ blockade or high Ca2+ permeability (PCa/PCs approximately 7.0). 4. PCa/PCs in the presence of KA decreased from 2.8 in the 1-day-old (1D) rat neurones to 1.1 and 0.44 in the 2-week-old (2W) and 6-month-old (6M) rat neurones, respectively. The concentration-response relationship for KA shifted to the left with age. The KA response was not affected by NS-102, a KA-selective antagonist, thus indicating that the increased affinity of the receptor for the ligand resulted from the change in the AMPA receptor channel subunits. 5. The AMPA response in the presence of 10(-4) M cyclothiazide showed a change in the inward rectifying current-voltage relationship with age. The KA response was strongly cross-desensitized by the addition of AMPA and was also blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), whereas a rapid desensitization of the AMPA response was removed in a concentration-dependent manner by cyclothiazide. These results indicate that the non-NMDA receptor channels are assembled from the subunits of the AMPA receptor family without the GluR-2 subunit, thus resulting in a high Ca2+ permeability. 6. The L-glutamate (Glu)-induced responses were more sensitive to DL-2-amino-5-phosphonopentanoic acid (APV) in the 1D rat neurones than in the adult rat neurones. 7. Both NMDA and KA raised the intracellular Ca2+ concentration ([Ca2+]i) in all neurones of 1D, 2W and 6M rats, though the charybdotoxin-sensitive Ca(2+)-activated K+ current (IK(Ca)) did not appear in the 1D rat neurones. An age-related prolongation of both IK(Ca) decay and [Ca2+]i clearance was also seen after the removal of KA. 8. It was thus concluded that the age-related changes of ionotropic receptors appear to play a key role in the activities of immature and mature rat Meynert cholinergic neurones. The KA-induced IK(Ca), which developed with ageing, may thus function as one of the negative feedback systems, and thereby prevent excess cell excitation and neural damage, especially in adult rats.

Anatomical and synaptic substrates for avian song learning

In songbirds, vocal learning occurs during periods of major cellular and synaptic change. This neural reorganization includes massive synaptogenesis associated with the addition of new neurons into the vocal motor pathway, as well as pruning of connections between song regions. These observations, coupled with behavioral evidence that song development requires NMDA receptor activation in specific song nuclei, suggest that experiences associated with vocal learning are encoded by activity driven, Hebbianlike processes of synaptic change akin to those implicated in many other forms of developmental plasticity and learning. In this review we discuss the hypothesis that develpmental and/or seasonal changes in NMDA receptor function and the availability of new synapses may modulate thresholds for plasticity and thereby define sensitive periods for vocal learning.

Hypoxia-induced dysfunction in developing rat neocortex

Neocortical slices from young [postnatal day (P) 5-8], juvenile (P14-18), and adult (>P28) rats were exposed to long periods of hypoxia. Field potential (FP) responses to orthodromic synaptic stimulation, the extracellular DC potential, and the extracellular Ca2+ concentration ([Ca2+]o] were measured simultaneously in layers II/III of primary somatosensory cortex. Hypoxia caused a 42 and 55% decrease in the FP response in juvenile and adult cortex, respectively. FP responses recorded in slices from young animals were significantly more resistant to oxygen deprivation as compared with the juvenile (P < 0.01) and adult age group (P < 0.001) and declined by only 3% in amplitude. In adult cortex, hypoxia elicited, after 7 +/- 4.5 min (mean +/- SD), a sudden anoxic depolarization (AD) with an amplitude of 14 +/- 6 mV and a duration of 0.89 +/- 0.28 min at half-maximal amplitude. Although the AD onset latency was significantly longer in P5-8 (12.5 +/- 4.9 min, P < 0.001) and P14-18 (8.7 +/- 3.2 min, P < 0.002) cortex, the amplitude and duration of the AD was larger in young (45.7 +/- 7.6 mV, 2.19 +/- 0.71 min, both P < 0.001) and juvenile animals (29.9 +/- 9.1 mV, P < 0.001, 0.96 +/- 0.26 min, P > 0.05) when compared with the adults. The hypoxia-induced [Ca2+]o decrease was significantly (P < 0.002) larger in young cortex (1,115 +/- 50 microM) as compared with the adult (926 +/- 107 microM). Prolongation of hypoxia after AD onset for >5 min elicited in young and juvenile cortex a long-lasting AD with an amplitude of 40.5 mV associated with a decrease in [Ca2+]o by >1 mM. On reoxygenation, only slices from these age groups showed spontaneous repetitive spreading depression in 3 out of 26 cases. In adults, the same protocol caused a significantly (P < 0.05) smaller and shorter AD and never a spreading depression. However, recovery in synaptic transmission after this long-term hypoxia was better in young and juvenile cortex, indicating a prolonged or even irreversible deficiency in synaptic function in mature animals. Application of ketamine caused a 49% reduction in the initial amplitude of the AD in juvenile cortex but did not significantly affect the AD in slices from adult animals. These data indicate that the young and juvenile cortex tolerates much longer periods of oxygen deprivation as compared with the adult, but that a sufficiently long hypoxia causes severe pathophysiological activity in the immature cortex. This enhanced sensitivity of the immature cortex is at least partially mediated by activation of N-methyl-D-aspartate receptors.

Retinal activity regulates developmental switches in functional properties and ifenprodil sensitivity of NMDA receptors in the lateral geniculate nucleus

Previous studies have shown that marked changes occur in the kinetic properties of N-methyl-D-aspartate (NMDA) receptors during development of the visual pathways. In the lateral geniculate nucleus (LGN) of the ferret, excitatory postsynaptic currents (EPSCs) induced by activation of NMDA receptors display a very slow decay time during the first postnatal month, then become shorter in duration following eye-opening (around postnatal day 32; P32). In view of the critical role that NMDA receptors play in activity-dependent refinement of visual connections during development, we have examined the mechanisms that underlie these changes and how they are regulated. To examine the role of retinal activity, whole-cell recordings were conducted in the LGN slice preparation obtained from normal ferrets and ferrets treated with continuous intraocular application of tetrodotoxin (TTX) from P25 until the time of recording. Blockade of ganglion cell activity with TTX prevented the changes in decay rate of the postsynaptic current induced by NMDA receptors. Treated animals older than P40 had NMDA-EPSCs markedly longer in duration than normal animals at a similar age, resembling responses present in normal newborn animals. To examine whether changes in subunit composition of the NMDA receptor may contribute to the maturation of its kinetic properties, we used the antagonist ifenprodil, which produces selective inhibition of heteromeric NMDA receptors containing the NR-2B subunit. Ifenprodil induced profound inhibition of NMDA receptor activity in normal young animals and TTX-treated mature animals, but substantially less inhibition in normal mature animals. These findings indicate that retinal activity is required for the developmental switch from a juvenile form of the NMDA receptor to a more mature form, possibly affecting NR2 subunit expression.

Effect of chronic administration of NMDA antagonists on synaptic development

The current research assessed the role of the N-methyl-D-aspartate (NMDA) receptor in developmental synaptic plasticity. This was accomplished by quantitative analysis of synaptic number and morphology following pharmacological manipulation of NMDA receptor activity using either the competitive antagonist 2-amino-5-phosphonovaleric acid (APV) or the noncompetitive antagonist phencyclidine (PCP). In the first group, 15-day-old male Long-Evans rats were implanted with osmotic minipumps, which administered 50 mM APV or vehicle at a rate of 0.5 microliter per h into the subjects' occipital cortex for 14 days. At age 30 days (P30), the rats were sacrificed and their occipital neocortices were examined. A second group of rats was given subcutaneous injections of 10 mg/kg PCP or vehicle once daily beginning on P5 for a period of 15 days, and was sacrificed on P20. To determine the effects following withdrawal from long-term NMDA antagonism, a third group of animals was given the same PCP injection routine until P20, but was sacrificed on P21, P26, P36, and P56. Developmental administration of APV was associated with a decreased molecular layer depth and estimated total number of synapses. Similarly, PCP induced a reduction in brain weight, molecular layer depth, and estimated total number of synapses. Withdrawal from NMDA antagonism was initially associated with similar results, i.e., reduced brain weight, cortex depth, synaptic density, and estimated total number of synapses, along with an increase in synaptic length. By P36, however, there was a transitory rebound associated with increased molecular layer depth and estimated total number of synapses. These results support the suggestion that NMDA receptor activation is integral to naturally occurring developmental synaptogenesis, and underscore the importance of NMDA receptor involvement in the process of synaptic plasticity.

Neural correlates of sensitive periods in avian song learning

A wide range of neural events takes place during avian song learning, including changes in receptor systems and synaptic organization and the wholesale production and growth of new cells and their connections. Any (or all) of these events may be involved in song learning, and therefore their timing may restrict when auditory memories are laid down in the song system and when these memories are used to pattern vocal behavior. In particular, mounting evidence indicates that NMDA receptor activation participates in the acquisition of auditory memories during song learning, and developmental changes in the number and function of NMDA receptors within the song system may regulate the capacity for song learning. These developmental changes in NMDA receptors occur in conjunction with a period of synapse elimination, and NMDA receptor activation during song learning may regulate this regressive event. In addition, both developmental and seasonal periods of vocal learning correlate well with neuronal addition and turnover within the vocal motor pathway. This temporal overlap provides the opportunity for auditory experience to shape the initial organization of naive motor circuitry. Thus, a variety of cellular substrates present themselves as candidates for providing the neural plasticity necessary for song learning and may together regulate how and when experience is stored in the nervous system.

Postnatal changes in the laminar and subcellular distribution of NMDA-R1 subunits in the cat visual cortex as revealed by immuno-electron microscopy

Although it is recognized that nearly all synapses in the cerebral cortex form postnatally, little is known about the emergence of molecules necessary to render these synapses functional. This study visualized the emergence of synaptically localized NMDA receptors by immuno-electron microscopic labeling of the receptor's obligatory subunit, NMDA-R1, in the developing cat visual cortex. Prior to eye-opening (postnatal day 2-10), NMDA-R1 immunoreactivity is already present within dendritic and growth cones, even though these profiles are devoid of synaptic specializations. This indicates that synthesis and incorporation of NMDA-R1 into plasma membranes are independent of form vision. During the next 2-3 weeks, i.e., preceding the onset of the critical period for ocular dominance plasticity (around the fourth week), NMDA-R1 immunoreactivity changes from a diffuse distribution within dendrites to a more discrete aggregation over postsynaptic densities of axo-spinous junctions. Such clustering of NMDA-R1 at synapses may be a prerequisite for stabilization and strengthening of synapses activated by visual stimulation during the critical period. Furthermore, only during the first several weeks, intensely NMDA-R1-immunoreactive neurons are present in the infragranular layers and the white matter. Enrichment of NMDA-R1 in the deep-layer neurons may reflect the neurons' supportive role in the development of cortical circuitry, serving as transient synaptic targets for geniculate and cortico-cortical afferents while these afferents 'wait' in the infragranular for their ultimate, life-long target neurons to become receptive in the upper layers.

Epilepsy in the developing brain: lessons from the laboratory and clinic

Children with epilepsy present unique challenges to the clinician. In addition to having differences in clinical and EEG phenomena, children differ from adults in regard to etiological factors, response to antiepileptic drugs (AEDs), and outcome. It is now recognized that the immature brain also differs from the mature brain in the basic mechanisms of epileptogenesis and propagation of seizures. The immature brain is more prone to seizures due to an imbalance between excitation and inhibition. gamma-Aminobutyric acid (GABA), the major CNS inhibitory neurotransmitter in the mature brain, can lead to depolarization in the hippocampal CA3 region in very young rats. There are also age-related differences in response to GABA agonists and antagonists in the substantia nigra, a structure important in the propagation of seizures. These age-related differences in response to GABAergic agents provide further evidence that the pathophysiology of seizures in the immature brain differs from that in the mature brain. Although prolonged seizures can cause brain damage at any age, the extent of brain damage after prolonged seizures is highly age dependent. Far less histological damage and fewer disturbances in cognition result from prolonged seizures in the immature brain than from seizures of similar duration and intensity in mature animals. However, detrimental effects of AEDs may be greater in the immature brain, than in the mature brain. These lessons from the animal laboratory raise questions about the appropriateness of current therapeutic approaches to childhood seizure disorders.

Differential distribution of AMPA receptors and glutamate during pre- and postnatal development in the visual cortex of ferrets

Immunohistochemical methods were used to study the distribution and time-course of appearance of cells expressing glutamate and alpha-amino-3-hydroxy-5-methyl-4-isoaxazole propionic acid (AMPA)-type glutamate receptors (GluR1 and GluR2/3) during development of the ferret visual cortex. Glutamate is present in many neurons in the ventricular zone, intermediate zone, developing cortical plate, and marginal zone as early as embryonic day (E) 34 (birth is at E41 in ferrets). Glutamate attains its adult distribution coincident with the completion of cellular migration. By contrast, GluR1 immunoreactivity emerges more slowly. By birth, GluR1 immunoreactivity is present only in a few neurons in the marginal zone and ventricular zone but is abundant in the marginal zone and subplate, where synaptogenesis commences. The number and staining intensity of GluR1-positive cells increases dramatically during the first two postnatal weeks and is maximal between the second and third week, before slowly declining to adult levels. Cortical cells immunopositive for GluR2/3 follow a similar pattern, although their distribution differs: GluR2/3-positive cells are mainly pyramidal cells. During the first postnatal week, GluR2/3 is also transiently present in fibers in the intermediate zone, which at this stage contains many thalamocortical and callosal and corticofugal axons. The abundance of glutamate at fetal stages, especially in the ventricular zone, is consistent with the previously proposed role of glutamate in mediating trophic effects in vivo, as previously demonstrated in vitro. The expression of AMPA receptors, as well as their transient overexpression, confirms the results of in situ hybridization studies and may imply a developmental role in neuronal differentiation for these receptors, in addition to their mature role in mediating cortical transmission.

Unilateral eyeball enucleation differentially alters AMPA-, NMDA- and kainate glutamate receptor binding in the newborn rat brain

The aim of the present work was to evaluate the neurochemical effects of early unilateral visual deprivation as a model of impaired visual maturation. For this purpose, binding to the different ionotropic glutamate receptor subtypes was quantified in vision-related and vision-unrelated brain structures of control and unilaterally deprived newborn rats. At post-natal (PN) day 10, male Sprague-Dawley rats underwent either unilateral eyeball enucleation (enucleation group, n = 12) or sham operation (control group, n = 12). In each group, brains were obtained either at post-natal day 20 (n = 6) or post-natal day 30 (n = 6) and processed for quantitative in vitro autoradiography selective for NMDA, kainate, and AMPA glutamate-binding sites, as well as for the presynaptic adenosine A1 receptor as a control of the deafferentation efficacy. In control animals, quantitative autoradiography revealed an increase in NMDA (e.g. +45% in superior colliculus) and kainate receptor binding (e.g. +55% in visual cortex, layer IV) from post-natal day 20 to post-natal day 30, associated with stable levels of AMPA receptor binding, in the vision-related structures. In the deafferented visual structures, monocular enucleation induced a marked decrease in A1 site density (e.g. -38 to -52%, in the superficial layer of superior colliculi, at PN day 20 and PN day 30, respectively) in parallel with a mild increase in both NMDA (e.g. +8 to 9%, in superior colliculi and visual cortex, layer IV at PN day 30, respectively) and AMPA (e.g. +16%, in layer IV of the visual cortex at PN day 30). Superimposed on marked bilateral decreases at PN day 30 in the enucleated rats, kainate receptor binding also revealed a slight but significant decrease (-5%) in the deafferented superior colliculus as compared to the non-deafferented side. The present findings (different time-courses of, and differential effects of deafferentation on, the NMDA, kainate and AMPA glutamate receptor subtypes throughout the visual brain structures) further support the involvement of these receptors in distinctive roles during maturation of the visual system.

Developmental down-regulation of LTD in cortical layer IV and its independence of modulation by inhibition

For in vitro LTD to remain viable as a model for synaptic weakening in visual cortical plasticity, it is crucial that it display a critical period for its induction within layer IV. A complicating factor, however, is that LTD in layer IV is modulated by inhibitory postsynaptic potentials (IPSPs); postsynaptic responses characterized as containing IPSPs do not depress in response to 1 Hz afferent stimulation. By blocking IPSPs intracellularly, we find that the ability to induce LTD in layer IV neurons is restored in juvenile, but not in mature animals. This developmental down-regulation of LTD induction is specific for layer IV when compared with LTD induction in layers II/III. These data are consistent with the hypothesis that an LTD-like phenomenon is involved in critical period plasticity and is apparently independent of developmental changes in inhibitory circuitry.

Postnatal changes in NMDAR1 subunit expression in the rat trigeminal pathway to barrel field cortex

N-methyl-D-aspartate (NMDA) type glutamate receptors are constituted of one obligatory subunit (NR1), expressed as eight splice variants, combined with one or more of four NMDAR2 subunits. Polyclonal antibodies were produced to an N-terminal domain of the NR1 subunit that recognize all eight splice variants. The antibody was used to localize NR1 in the trigeminal pathway to barrel field cortex in rats. The distribution and density of NR1 changes between birth (postnatal day 0 = P-0) and P-360. The trigeminal nuclei already contain a high level of NR1 immunoreactivity on the day of birth. The ventral posterior lateral, ventral posterior medial, and posterior nucleus, medial division, thalamic nuclei show fluctuations in NR1 immunoreactivity levels, starting at birth with moderate densities in neuropil which decrease at P-7, and peak again in neuronal cell bodies as well as the neuropil at P-21. In the cortex, the density of NR1 in layer VI fluctuates with low points at P-7 and P-40. Superficial cortical layers I, II, and III reach adult levels at P-14 and remain high. NR1 levels decrease sharply in layer IV just prior to P-40 and then slowly recover over the next 3 months to stabilize at moderate levels in the adult. In addition to neuronal expression there is a transient high level of labeling in glial cells with a peak density of staining at P-21. The results emphasize that NR1 subunit expression is finely regulated in rat somatic sensory pathways for periods as long as 7-8 weeks after birth in the barrel field cortex.

Ischemia and lesion induced imbalances in cortical function

Cortical structures are often critically affected by ischemic and traumatic lesions which may cause transient or permanent functional disturbances. These disorders consist of changes in the membrane properties of single cells and alterations in synaptic network interactions within and between cortical areas including large-scale reorganizations in the representation of the peripheral input. Prominent functional modifications consisting of massive membrane depolarizations, suppression of intracortical inhibitory synaptic mechanisms and enhancement of excitatory synaptic transmission can be observed within a few minutes following the onset of cortical hypoxia or ischemia and probably represent the trigger signals for the induction of neuronal hyperexcitability, irreversible cellular dysfunction and cell death. Pharmacological manipulation of these early events may therefore be the most effective approach to control ischemia and lesion induced disturbances and to attenuate long-term neurological deficits. The complexity of secondary structural and functional alterations in cortical and subcortical structures demands an early and powerful intervention before neuronal damage expands to intact regions. The unsatisfactory clinical experience with calcium and N-methyl-D-aspartate antagonists suggests that this result might be achieved with compounds that show a broad spectrum of actions at different ligand-activated receptors, voltage-dependent channels and that also act at the vascular system. Whether the same therapy strategies developed for the treatment of ischemic injury in the adult brain may be applied for the immature cortex is questionable, since young cortical networks with a high degree of synaptic plasticity reveal a different response pattern to hypoxic and ischemic insults. Age-dependent molecular biological, morphological and physiological parameters contribute to an enhanced susceptibility of the immature brain to these noxae during early ontogenesis and have to be investigated in more detail for the development of adequate clinical therapy.

A discussion of activity-dependent forms of synaptic weakening and their possible role in ocular dominance plasticity

Activity-dependent synaptic weakening is likely to be involved in numerous types of developmentally regulated cortical plasticity. The possible roles of two models of synaptic weakening, homosynaptic- and heterosynaptic-long-term-depression (LTD), are discussed. Metabotropic glutamate receptors (mGluRs) as they relate to LTD and ocular dominance plasticity will also be considered.

Glutamate receptors and development of the visual cortex: effect of metabotropic agonists on cAMP

Glutamate receptors are more active in several respects in young animals than in adults. Here we examine the effect of metabotropic glutamate agonists on rat cortical cAMP during and after the critical period for visual cortex plasticity. Quisqualate produced a substantial increase in cAMP, which was larger during the critical period than in the adult. The increase was not affected by CNQX or APV, showing that it was not due to the action of quisqualate on ionotropic glutamate receptors. Both Type I mGluRs (mGluRs 1 and/or 5) and Type II mGluRs (mGluRs 2 and/or 3) probably contributed to the cAMP increase because (i) ACPD and L-CCG-I, which are more active on Type II mGluRs, were more effective than DHPG, which is more active on Type I mGluRs; and (ii) there was a significant difference in the effect of ACPD on the increase in cAMP, comparing mGluR1 knockout mice with control mice. Agonists which produce large stimulation of cAMP production (ACPD, L-CCG-I), as well as L-AP4, also produced small attenuations of forskolin-stimulated cAMP, but only at high concentrations. Thus, we conclude that it is the stimulation and/or potentiation of cAMP production that is significant, rather than the attenuation of forskolin-stimulated cAMP. Since this stimulation and/or potentiation is higher during the critical period than in the adult, and the cAMP second messenger system has been implicated in hippocampal plasticity, it may also play a role in visual cortex plasticity.

Pre- and postnatal expression of amino acid neurotransmitters, calcium binding proteins, and nitric oxide synthase in the developing superior colliculus

Neurons within the superior colliculus (SC) contain a variety of neurochemicals, including the amino acid neurotransmitters GABA and glutamate, the calcium binding proteins calbindin and parvalbumin, and the neuromodulator nitric oxide. We have examined the development of expression of these substances using antibody immunocytochemistry. These results are summarized in Fig. 10. GABA and calbindin are expressed very early in development, at a time when cells are still dividing and migrating from the subventricular zone. The expression of both GABA and CB is maximal at around E40-46, the age at which these cells have just established their adult lamination and extrinsic afferents have begun to grow into the tectum. GABA and CB likely play diverse roles during this stage of development, including the regulation of intracellular calcium during cell migration and neurite outgrowth. Glutamate is expressed somewhat later in development while parvalbumin immunoreactivity does not appear until shortly after birth. These two substances continue to increase in density throughout the period of postnatal growth, at a time when synapse formation and evoked electrical activity are beginning to develop. Both PV and glutamate may be involved in one or both of these activity-dependent processes. Nitric oxide synthase (NOS) is expressed at different times in different cell groups. NOS appears very early in prenatal development in cells within the SVZ and in the deep gray layer of SC. On the other hands, cells within the intermediate gray layer of SC do not express NOS until shortly before birth. The igl cells that express NOS at this age are clustered neurons similar to those that project to the CFR in the adult. NOS expression occurs in these cells at precisely the time when axons begin to form patches that innervate these clusters. Based upon this temporal correlation, we hypothesize that nitric oxide may regulate synapse formation in this cell group.

Development and plasticity of cortical processing architectures

One of the basic functions of the cerebral cortex is the analysis and representation of relations among the components of sensory and motor patterns. It is proposed that the cortex applies two complementary strategies to cope with the combinatorial problem posed by the astronomical number of possible relations: (i) the analysis and representation of frequently occurring, behaviorally relevant relations by groups of cells with fixed but broadly tuned response properties; and (ii) the dynamic association of these cells into functionally coherent assemblies. Feedforward connections and reciprocal associative connections, respectively, are thought to underlie these two operations. The architectures of both types of connections are susceptible to experience-dependent modifications during development, but they become fixed in the adult. As development proceeds, feedforward connections also appear to lose much of their functional plasticity, whereas the synapses of the associative connections retain a high susceptibility to use-dependent modifications. The reduced plasticity of feedforward connections is probably responsible for the invariance of cognitive categories acquired early in development. The persistent adaptivity of reciprocal connections is a likely substrate for the ability to generate representations for new perceptual objects and motor patterns throughout life.

Transient co-localization of calretinin, parvalbumin, and calbindin-D28K in developing visual cortex of monkey

This paper reports a double-labelling immunocytochemical study of the three calcium-binding proteins calretinin, parvalbumin, and calbindin-D28k in developing and adult Macaca primary visual cortex. In adult visual cortex, each protein marks a subset of GABAergic neurons with a characteristic laminar distribution and virtually no co-localization was found between these three proteins, suggesting that each calcium-binding protein may serve as a marker for one or more cortical subcircuits. The immature visual cortex, immunostained using identical techniques was then analysed to determine if each calcium-binding protein could serve as a developmental marker for these circuits. The Cajal-Retzius cells of layer 1 contained all three proteins during development. Calbindin-D28k and calretinin were co-localized starting at Fd (foetal day) 45 and after Fd125, parvalbumin also was present in the same Cajal-Retzius cells. All three proteins continued to be expressed until the Cajal-Retzius disappeared postnatally. In layers 2-6 calbindin-D28k and calretinin were never co-localized. In contrast, parvalbumin and calretinin were found in neurons of deep layer 3 from Fd 155 to postnatal (P6) weeks with a few persisting even later. Before birth almost all PV+ neurons in layers 4-6 were CaB+, but by P3 weeks only a few PV+/CaB+ neurons remained in layer 4C and these completely disappeared by P6 weeks. Co-localization in layer 4 neurons overlaps the period of ocular dominance segregation, suggesting that the onset of cortical maturity coincides with segregation of calcium-binding proteins within the GABA interneurons.

NMDA receptors in mouse barrel cortex during normal development and following vibrissectomy

The development of N-methyl-D-aspartate (NMDA) receptors and the effects of vibrissectomy upon [3H]MK-801 binding were examined in the barrel cortex of mice. Autoradiographic studies showed that initially very low binding of [3H]MK-801 sharply increased during the second postnatal week reaching the adult level by the end of the third week. Scatchard analysis performed on cortical membrane preparations indicated that this rise of [3H]MK-801 labelling was due to an increase in the number of binding sites and a decrease of Kd at postnatal day 15 and 28. The interlaminar differences of labelling were registered from postnatal day 8. Changes of interlaminar distribution were found during the second and third postnatal weeks. In adult barrel cortex the highest binding was found in supragranular layers. In layer IV of the cortex, the pattern of binding resembled the pattern of barrels. Unilateral denervation of vibrissae performed in neonatal and adult mice did not alter the intensity of [3H]MK-801 labelling or the laminar distribution of binding sites. These results suggest that NMDA receptor binding does not reflect the plastic changes occurring in the barrel cortex.

Co-regulation of long-term potentiation and experience-dependent synaptic plasticity in visual cortex by age and experience

Long-term potentiation (LTP) is a lasting enhancement of excitatory synaptic transmission that follows specific patterns of electrical stimulation. Although the mechanism of LTP has been intensively studied, particularly in the hippocampus, its significance for normal brain function remains unproven. It has been proposed that LTP-like mechanisms may contribute to naturally occurring, experience-dependent synaptic modifications in the visual cortex. The formation of normal binocular connections within the visual cortex requires simultaneous input from both eyes during a postnatal critical period that can be delayed by rearing animals in complete darkness. To explore the role of LTP in this experience-dependent maturation process, we induced LTP in visual cortical slices taken at different ages from light-reared and dark-reared rats. Susceptibility to LTP coincides with the critical period and, like the critical period, can be prolonged by rearing animals in darkness. These findings support the hypothesis that LTP reflects a normal mechanism of experience-dependent synaptic modification in the developing mammalian brain.

Early isolation from conspecific song does not affect the normal developmental decline of N-methyl-D-aspartate receptor binding in an avian song nucleus

Early effects of experience on synaptic reorganization and behavior often involve activation of N-methyl-D-aspartate (NMDA) receptors. We have begun to explore the role of this glutamate-receptor subtype in the development of learned birdsong. Song learning in zebra finches occurs during a restricted period that coincides with extensive synaptic reorganization within neural regions controlling song behavior. In one brain region necessary for song learning, the lateral magnocellular nucleus of the anterior neostriatum (IMAN), NMDA receptor binding is twice as high at the onset of song learning as in adulthood. In the present study, we used quantitative autoradiography with the noncompetitive NMDA antagonist [3H]MK-801 to examine more closely the developmental decline in NMDA receptor binding within IMAN and found that it occurred gradually over the period of song learning and was not associated with a particular stage of the learning process. In addition, early isolation from conspecific song did not affect [3H]MK-801 binding in IMAN at 30, 60, or 80 days. Since behavioral studies confirmed that our isolate rearing conditions extended the sensitive period for song learning, we conclude that the normal developmental decline in overall NMDA receptor binding within IMAN does not terminate the capacity for song learning. Finally, early deafening, which prevents both stages of song learning, also did not affect [3H]MK-801 binding in IMAN at 80 days, indicating that the decline in NMDA receptor binding occurs in the absence of auditory experiences associated with song development.

The ontogeny of glutamate receptors in rat barrel field cortex

The ontogeny of N-methyl-D-aspartate (NMDA) and non-NMDA excitatory amino acid receptors in rat barrel field cortex were characterized using receptor autoradiography. NMDA receptors showed a different pattern of development than that of non-NMDA receptors recognizing quisqualate (QUIS sites). During the first 14 days, high densities of QUIS sites were localized in barrel centers forming a sensory map of the rat whisker pad. After that time, the density of QUIS sites in barrel centers decreased so that the pattern was no longer apparent by postnatal day 21. In contrast to QUIS sites, NMDA sites did not exhibit a somatotopic pattern until postnatal day 21, when the lower density of sites in barrel septa formed an outline of barrel centers. At all ages examined, the density of NMDA sites did not differ significantly between barrel centers and surrounding cortex. Of the non-NMDA receptors examined in the postnatal day 10 old rat, both metabotropic sites and the NNKQ sites, which are [3H]glutamate binding sites that are not displaceable by NMDA, kainate or QUIS, showed a pattern of higher densities in barrel centers than surrounding tissue, whereas AMPA sites exhibited a complementary pattern. [3H]Glutamate binding to metabotropic sites was not significantly displaced by QUIS, whereas both NNKQ sites and metabotropic sites were potently blocked by the metabotropic agonist trans-ACPD. These results suggest that the NNKQ sites are low affinity QUIS metabotropic receptors, which, due to their high density in the immature barrel field, are in a position to influence barrel formation.

Developmental alterations in the sensitivity of hippocampal NMDA receptors to AP5

Changes in the properties of the N-methyl-D-aspartate (NMDA) receptor are proposed as a factor in the decrease in synaptic plasticity during maturation of the brain. Alterations in the hippocampal NMDA receptor population were studied during development by comparing competitive antagonist efficacy during a window characterized by hyperexcitability and active synaptogenesis to that seen in a more mature period. A developmental change in the sensitivity of the N-methyl-D-aspartate (NMDA) evoked response to the competitive antagonist D(-)2-amino-5- phosphonopentanoic acid (D-AP5) was observed by whole-cell mode voltage-clamp, intracellular and extracellular recordings in hippocampal slices. These differences were observed in the portions of the hippocampus that contain the terminal axon arbors of the CA3 pyramidal neurons, the synaptic fields of the Schaffer collateral, associational and commissural pathways. The apparent antagonist efficacy was 2-3 times greater in immature slices obtained on postnatal day (PND) 10-16 than that observed in more mature tissue (> PND38). Given the possible role of the NMDA receptor in network plasticity, these observations could indicate a role for the molecular diversity of this important receptor subtype family in the maturation of hippocampal pathways.

Chronic neonatal phencyclidine treatment produces age-related changes in pentylenetetrazol-induced seizures

Although excitatory amino acids are known to play a critical role in the plasticity of developing brain, the behavioral effects of blocking the N-methyl-D-aspartate (NMDA) receptor-gated ion channel during development are not clear. Here we report the effects of chronic postnatal administration of 1-phenylcyclohexylpiperidine (phencyclidine or PCP), a NMDA channel blocker, on seizure susceptibility. To study the short-term effects of chronic PCP administration on pentylenetetrazol (PTZ)-induced seizures, rats were treated with PCP (5 mg/kg, i.p.) for 11 days from postnatal days 5-15, 24-34 or 44-54 and tested in the PTZ-induced seizure paradigm on postnatal days 21, 40 and 60, respectively. Administration of PCP in 5-15-day-old rats resulted in increased seizure susceptibility at day 21, while administration of PCP in postweanling rats (days 24-34) markedly attenuated their susceptibility to seizures at day 40. PCP injection had little effect on the seizure susceptibility of older rats. To study the long-term effects of postnatal PCP treatment, rats were injected with PCP (5 mg/kg from postnatal day 5-15, i.p.) and were tested for PTZ-induced seizures on postnatal days 40 and 60; each rat was tested only once. When tested for PTZ-induced seizure on day 40, PCP-treated rats did not differ from saline-treated controls. When tested on day 60, PCP-treated rats had a lower incidence of seizures and in the rats that did have seizures their latencies were significantly prolonged compared to controls. Together, our data suggest that chronic PCP administration alters PTZ-induced seizure susceptibility in an age-dependent manner and chronic PCP administration in postnatal rats produces long-term changes that persist into adulthood.

Distinct laminar differences in the distribution of excitatory amino acid receptors in adult ferret primary visual cortex

In order to explore the relative contributions of the different ionotropic excitatory amino acid receptor subtypes to signalling in primary visual cortex, we have mapped their distributions in area 17 of adult ferret cerebral cortex by quantitative in vitro autoradiography. D,L-alpha-amino-3-hydroxy-5-methoxy-4-isoxazole propionate (AMPA) and kainate receptors, gating fast, Na(+)-permeable channels, were localized with [3H]dizocilpine maleate ([3H]MK-801). All three radioligands bound to single sites, with KDs of 414 nM [3H]AMPA and [3H]kainate, respectively. Slower-acting N-methyl-D-aspartate receptors, which gate the influx of Ca2+ as well as Na+, were localized with ([3H]AMPA), 78 nM ([3H]kainate) and 16 nM ([3H]MK-801), and each receptor subtype displayed a different laminar distribution pattern within area 17. AMPA receptors were concentrated in superficial layers, with intermediate densities in deep layers and lowest levels in layer IV. Kainate receptor levels were high in layers V and VI and low in all other layers. N-methyl-D-aspartate receptors were more homogeneously distributed than AMPA or kainate receptors, but were expressed at highest levels in layers I and IV and lowest levels in layers V and VI. The binding site densities found in the layers containing most receptors were Bmax = 2812 fmol/mg for [3H]AMPA, Bmax = 626 fmol/mg for [3H]MK-801 maleate and Bmax = 278 fmol/mg for [3H]kainate. Thus, while AMPA receptors were predominant and kainate receptors least abundant in all cortical layers, a complementary relative distribution of excitatory amino acid receptors was apparent, with AMPA receptor density highest in superficial layers, kainate receptor density highest in inferior layers and N-methyl-D-aspartate receptor density highest in the middle granular layer, as well as in layer I. The results indicate that although AMPA receptors are principally involved in excitatory signalling in adult ferret primary visual cortex, kainate receptors in layers V and VI and N-methyl-D-aspartate receptors in layers I and IV may have particularly important roles in mediating synaptic transmission.